The invention relates to a fiber-optic cable having at least one element for high tensile stress, at least one cable cladding, and at least one element with a chamber receiving at least one optical transmission element having outer dimensions slightly smaller than the dimensions of the chamber.
It is known that optical fiber elements have different thermal coefficients of expansion than the other elements of the cable, particularly the cable sheath, cladding materials of the leads, possible resistance elements against tension or crushing, and so on. When the optical fibers are accommodated loosely in a chamber, then they are coupled to the actual cable solely by friction, and as a consequence of the different thermal coefficients of expansion of the actual cable compared to the optical fiber element, macrobends can arise, which result in an increase in attenuation for the optical fiber element. It is therefore common to give the chamber that accepts the optical fiber elements such ample dimensions that elevations of attenuation due to macrobends are avoided. This very broad method has the disadvantage that the cable cross-section is significantly increased, because the cables known hitherto have always involved some type of xe2x80x9coverdimensioningxe2x80x9d. Additional details of the cable structure, dimensioning and calculation can be found in chapter 9 entitled xe2x80x9cOptical Cable Designxe2x80x9d, Fiber Optic Cables, G. Mahlke and P. Gxc3x6ssing; John Wiley and Sons Ltd., 3rd ed., 1997, pp. 115-158.
U.S. Pat. No. 4,770,489 teaches the loose accommodating of optical fibers in corresponding chambers or tubes. A plurality of tension-resistant elements are additionally provided, which comprise a higher E module than the actual cable and at the same time, a lower thermal coefficient of expansion. For example, fibers that consist of aramide yarn, carbon, or glass embedded in an epoxy resin can be used for these elements. In this way, the operative range of a cable can reach from xe2x88x9220 degrees Celsius to +70 degrees Celsius, instead of from xe2x88x9220 degrees to +60 degrees. This is assuming that the cable structure as such is left unaltered.
U.S. Pat. No. 5,098,177 teaches an optical cable that comprises an element for high tensile stresses. The optical fibers are accommodated loosely in chambers or tubes and comprise a coating of LCP (Liquid Crystal Polymer) whose linear temperature coefficient of expansion is between xe2x88x9215xc2x710xe2x88x926 and 5xc2x710xe2x88x926(1/K). In consideration of the different expansions, a relatively large tolerance range (play) of 0.5% to the wall is provided. This and the coating processes represent an additional outlay, and besides, the outer diameter of the optical fibers is increased by the added coating, so that a part of what is achieved by limiting the expansion behavior is lost again due to the increased space requirement of the optical fiber.
It is the object of the invention to demonstrate how it is possible to reduce, in a simple manner, the space requirement of the chamber within the optical cable that accepts the optical transmission element(s). This object is achieved by a cable having at least one element for high tensile stress, at least one cable cladding and at least one element with a chamber receiving at least one optical transmission element having an outer dimension slightly smaller than the dimension of the chamber and the cable has a thermal contraction (dL/L) in a range from 20xc2x0 C. to a lower temperature, for example xe2x88x9230xc2x0 C., which does not differ more than 30% from the thermal contraction of the optical transmission element.
The invention is based on tuning (synchronizing) the expansion behavior of the cable on one hand and of the optical transmission element on the other hand (in the form of an individual optical fiber, a band of optical fibers or some other elongated optical fiber structure forming a mechanical unit) to one another in the lower temperature rangexe2x80x94which is particularly critical for macrobendsxe2x80x94in such a manner that these deviate from each other only by a predetermined value at the most. In this way, it is possible to create an optimally adjusted xe2x80x9cmovement windowxe2x80x9d for the optical transmission element, this window being optimally small in and of itself, but nevertheless still sufficient to accept macrobends which result from different residual, in particular local, linear expansions (e.g. as a result of bending) and thereby to prevent elevations of attenuation.
In addition to the above solution or independent of it, the fiber-optic cable can also be inventively constructed such that, below 20xc2x0 C., particularly at the lower temperature limit value of the cable (e.g. xe2x88x9230xc2x0 C.,), the disparity between the percentage expansion value of the optical transmission element, on one hand, and the percentage expansion value of an appertaining cable structure, on the other hand, is selected such that the difference of the expansion values is less than xc2x10.03 percentage points, preferably less than xc2x10.02 percentage points, and particularly less than xc2x10.01 percentage points.
The dimensions of the chamber are appropriately so constructed that local bending radii that may result from an additional excess length of the optical transmission element at the lowermost temperature limit value are greater than 70 mm.
In addition to the above solution or independent of it, another course of action consists in thickening the optical transmission element using plastic additives that are connected to it in a mechanically secure manner (for instance, using an additional coating on the optical fiber and/or some other type of material deposition, for instance on the outside on optical fiber bands), such that at low temperatures a preferably slight (e.g. max. 0.05%) adjustment of the expansion to the expansion of the cable structure is achieved.
The invention and its developments are detailed below with the aid of drawings, which represent exemplifying embodiments.